With the trend towards higher levels of integration and further miniaturization of a semiconductor circuits, crystal defects formed at the time of crystal growth existing near the surface layer of a silicon wafer can have a great effect on device performance. In general, crystal defects that degrade device characteristics are of the following three kinds.    1. Void defects that occur as a result of aggregation of vacancies    2. Oxidation Induced Stacking Faults (OSF)    3. Dislocation clusters that occurs as a result of aggregation of interstitial silicon
In order to obtain a silicon wafer that does not include the above crystal defects formed at the time of crystal growth near the surface layer where a device circuit is manufactured, the following methods have been devised.    1.) To manufacture defect-free single crystal ingots by controlling the crystal growth conditions    2.) To eliminate void defects near the surface layer of the wafer by high-temperature annealing    3.) To grow a defect-free layer on the surface of a wafer by epitaxial growth
Although the above methods 1) to 3) can prevent the problem of crystal defects, none of these methods is necessarily preferable for the control of oxygen precipitates. Because oxygen precipitates (bulk micro defect: BMD) play an important role as gettering sites against harmful heavy-metal contamination incidentally occurring in the device circuit manufacturing process, they are preferably generated at an appropriate density in the heat treatment in the device circuit manufacturing process. However, the conditions selected for restricting crystal defects in the above methods 1) to 3) are often disadvantageous for the generation of BMDs. To begin with, BMD control in 1), 2) and 3) is described.
When a defect-free single crystal ingot in 1) is to be manufactured, the oxygen concentration needs to be lowered to prevent OSFs from occurring. However, under a low oxygen concentration conditions, it is difficult to obtain sufficient BMD density by a normal heat treatment. Also, the crystal in the defect-free single crystal ingot is in a state wherein a vacancies-dominant defect-free portion and an interstitial silicon-dominant defect-free portion coxexist in a radial direction, and the oxygen precipitate characteristics are totally different between these portions. Accordingly, a post-treatment to ensure uniform oxygen precipitate characteristics within the silicon wafer surface is need din some cases. As a post-treatment for uniform oxygen precipitate characteristics, a method of eliminating such precipitate characteristics in an as-grown state by a rapid high-temperature heating treatment has been proposed (for example, Patent Documents 1 and 2). Also, a method to make uniform the BMD density by a heat treatment that strongly effects nuclear generation on the low-temperature side that is not influenced by the dominant point defect kind has been proposed (for example, Patent Document 3 and 4). However, the former method requires a rapid high-temperature heating treatment, which is a high-cost process, and the latter method requires a long-time heat treatment process, by which it is possible to density and make uniform the BMDs but it is very difficult to control the density to a selected density. Thus, the defect-free single crystal ingot is actually used only for limited applications that do not require gettering by the BMDs.
Next, a high-temperature annealed wafer in 2) is described. Void defects near the surface are eliminated by a heat treatment typically for one hour at 1200° C. under a non-oxidizing atmosphere such as hydrogen and argon. However, the voids in a normal crystal that are eliminated by annealing are only those on the outermost surface layer of the wafer, and in order to create a void defect free area deeper than the device manufacturing area, it is very important to minimize the size of the voids so that they can be eliminated easily (for example, Patent Document 5 shown below). However, in order to create a void defect free area that is deep enough, merely minimizing the void defects is not sufficient, but lowering the oxygen concentration is also strongly desired (for example, Non-Patent Document 1 shown below). This is because the inner wall of the void is covered with an oxide film in an as-grown state, and the shrinkage of the void defects starts after the inner wall oxide film is dissolved and eliminated. That is, as the depth which is sufficient for the inner wall oxide film to be dissolved and eliminated by the oxygen outward diffusion effect depends on the oxygen concentration of the crystal, the depth is shallow in crystals with a high oxygen concentration. Thus, in order to obtain a sufficient void defect free layer depth, lowering the oxygen concentration of the crystal is very important. However, it is generally difficult for a crystal with a low oxygen concentration to have sufficient BMD density.
Further, an epitaxially grown wafer in 3) is described. The epitaxially grown wafer is advantageous in the voids present in the substrate wafer are not transcribed on the epitaxial layer, and a defect-free silicon layer can be obtained on the surface layer. However, since the wafer temperature is raised in a short time to a high temperature for epitaxial growth in consideration of the productivity, as-grown micro nuclei that becomes BMD nuclei disappear, and it is very likely that sufficient BMD density will not be obtained even if a heat treatment is applied thereafter.
In consideration of the above described situation concerning high-temperature annealed wafers and epitaxial wafers, a method of nitrogen doping has been proposed. First, the effect of nitrogen doping on the high-temperature annealed wafer is described. Using a crystal whose void defect size has been reduced by doping nitrogen as a crystal for high-temperature annealing has been proposed (for example, Patent Document 6 shown below). Also, although it is traditionally known that abnormal BMDs are generated in nitrogen-doped CZ silicon crystals, it has been shown that controlling the nitrogen concentration can result in BMDs with appropriate density (for example, Patent Document 7 shown below). This document shows that 1×109 units/cm3 or more BMD density can be obtained when the nitrogen concentration of even a crystal with low oxygen concentration is set to 1×1013 atoms/cm3 or more. Many similar inventions to this exist, but they describe only the nitrogen concentration as a factor to control the BMD density of a high-temperature annealed wafer (for example, Patent Documents 7, 8, 9 and 10 shown below). However, the BMD density is determined not only be nitrogen concentration but by the temperature increase rate in each temperature range at the time of high-temperature annealing (for example, Patent Document 11 shown below). However, the range defined in this document includes all condition ranges to be selected generally, and this document does not disclose a method for controlling the BMDs individually. Also, a method of obtaining an appropriate BMD density by performing annealing for 60 minutes or more at a temperature of 700° C. or more and 900° C. or less as a heat treatment before high-temperature annealing or by setting the thermal annealing rate in a temperature range from 700° C. or more to 900° C. or less to 3° C./minute or less in the thermal annealing step of high-temperature annealing has been proposed (for example, Patent Document 12 shown below). However, the BMDs are not controlled only by this definition, and a comprehensive method for controlling the BMDs has not been disclosed.
Next, a nitrogen doping technique proposed for an epitaxially grown wafer is described. As for the epitaxially grown wafer, it is likely that oxygen precipitate nuclei will disappear in the epitaxial growth process. In contrast, it if shown that 5×103 units/cm3 or more BMD density is observed even after epitaxial growth when doping with 1×1013 atoms/cm3 or more nitrogen (for example, Patent Document 13 shown below). this may be because, in the nitrogen-doped crystal, generation of as-grown nuclei starts during crystal growth at a higher temperature than in a normal crystal, and they grow at the high temperature and form large-sized nuclei. It is thought that these large-sized nuclei do not disappear even in the epitaxial growth process. However, this BMD density is not necessarily preferable. It is shown that, although the BMD density after epitaxial growth is greatly influenced by nitrogen concentration, is also influenced by the annealing process time at a high temperature before the epitaxial growth process (for example, Patent Document 14 shown below). The annealing process before the epitaxial growth means an H2 or HCl baking process at an equal or higher temperature than the epitaxial growth temperature generally for the purpose of elimination of natural oxide films. Meanwhile, it is shown that the BMD density observed after the epitaxial growth is influenced by the cooling rates in two temperature ranges, that is, a cooling rate from 1150° C. to 1020° C. and a cooling rate from 1000° C. to 900° C., in the crystal growth process (for example, Patent Document 15 shown below). The effects of the cooling rates in the two temperature ranges are as follows. The temperature range from 1150° C. to 1020° C. is a temperature range for the generation and growth of void defects, and since rapid cooling in this temperature range restrains the absorption of vacancies in voids, the concentration of residual vacancies is raised, and subsequent BMD nucleus generation is promoted. The temperature range from 1000° C. to 900° C. is regarded as a temperature range in which the BMDs are generated in a nitrogen-doped crystal, and slow cooling in this temperature range increases the BMD density, according to this document. This document recommends that the cooling rate from 1150° C. to 1020° C. be 2.7° C./minute or more, and the cooling rate from 1000° C. to 900° C. be 1.2° C./minute or less, in order to obtain sufficient BMD density. However, it says the effect of the cooling rate in the temperature range from 1000° C. to 900° C. is slight, and the effect of the cooling rate from 1150° C. to 1020° C. is significant. However, the relation of these with the nitrogen concentration and other factors is not described clearly (Patent Document 15 shown below). It has been proposed that the BMD density after the epitaxial growth is further controlled by performing pre-annealing for 15 minutes or more and 4 hours or less at a temperature of 700° C. or more and 900° C. or less before the epitaxial growth process (for example, Patent Document 16 shown below).
As described above, it is clear the BMD density of a nitrogen-doped silicon wafer after the epitaxial process depends on 1) the nitrogen concentration, 2) the crystal thermal history, 3) the temperature and time of a high-temperature heat treatment performed for elimination of natural oxide films in the epitaxial process, 4) the temperature and time of pre-annealing performed before the epitaxial process, and 5) the oxygen concentration, as a matter of course.
The BMD density of a nitrogen-doped silicon wafer after the high-temperature annealing process depends on 1) the nitrogen concentration, 2) the crystal thermal history, 3) the thermal annealing rate during the high-temperature annealing process, 4) the temperature and time of pre-annealing performed before the high-temperature annealing process, and 5) the oxygen concentration. Although it is known that the BMD density depends on these many factors simultaneously, the effect of these factors has only partially been clarified, as described above. As the nitrogen has a small segregation coefficient in the nitrogen-doped crystal, the nitrogen concentration exhibits significant changes in the axial direction of the crystal. Also, as the BMDs of the nitrogen-doped crystal depend on large as-grown oxygen precipitate nuclei generated in the cooling process during the crystal growth, they are strongly influenced by the thermal history during crystal growth. The growth process of the as-grown nuclei to visible size strongly influenced by nitrogen concentration and thermal history have not been clarified, and thus it has been conventionally necessary to control the nitrogen doping amount and the heat treatment process for obtaining an appropriate BMD density for each crystal growth condition and for each process.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2001-503009
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2002-299344
Patent Document 3: Japanese Unexamined Patent Application Publication No. 2000-264779
Patent Document 4: Japanese Unexamined Patent Application Publication No. 2002-134517
Patent Document 5: Japanese Unexamined Patent Application Publication No. H10-208987
Patent Document 6: Japanese Unexamined Patent Application Publication No. H10-98047
Patent Document 7: Japanese Unexamined Patent Application Publication No. 2000-211995
Patent Document 8: Japanese Unexamined Patent Application Publication No. H11-322491
Patent Document 9: Japanese Unexamined Patent Application Publication No. 2001-270796
Patent Document 10: Japanese Unexamined Patent Application Publication No. 2001-284362
Patent Document 11: Japanese Unexamined Patent Application Publication No. 2002-118114
Patent Document 12: Japanese Unexamined Patent Application Publication No. 2002-353225
Patent Document 13: Japanese Unexamined Patent Application Publication No. H11-189493
Patent Document 14: Japanese Unexamined Patent Application Publication No. 2000-044389
Patent Document 15: Japanese Unexamined Patent Application Publication No. 2002-012497
Patent Document 16: Japanese Unexamined Patent Application Publication No. 2003-73191
Non-Patent Document 1: K. Nakamura, T. Saishoji, and J. Tomioka; The 63rd JSAP (The Japan Society of Applied Physics) Annual Meeting Digest, Autumn 2002; P. 381; No. 1 24p-YK-4